Mimas: An Intriguing Interior

byPaul GilsteronOctober 20, 2014

I like what Radwan Tajeddine (Cornell University) has to say about recent work on Saturn’s moon Mimas. The lead author of a paper on the subject in Science, Tajeddine compares recent Cassini observations of the moon to a child shaking a wrapped gift, trying to figure out what the package conceals. ‘Shaking’ Mimas in a similar way through analysis of the Cassini data has revealed what might be a sub-surface ocean, or an unusually-shaped core preserved since the moon’s formation.

At work here is a technique called stereo photogrammetry, in which astronomers measure the moon’s libration around its polar axis. Libration is an oscillation or ‘wobble’ that can be studied by looking at Cassini imagery — taken by its Imaging Science Subsystem at different times and angles — and analyzing the images with the help of a computer model that involves hundreds of reference points on the surface. The amount of Mimas’ libration points to interesting things in the interior, but just what we still don’t know.

Image: Cassini came within about 9,500 kilometers (5,900 miles) of Mimas during its flyby on Feb. 13, 2010. This mosaic was created from seven images taken that day in visible light with Cassini’s narrow-angle camera. An eighth, lower-resolution image from the same flyby, taken with the wide-angle camera, was used to fill in the right of the mosaic. The images were re-projected into an orthographic map projection. This view looks toward the hemisphere of Mimas that leads in its orbit around Saturn. Mimas is 396 kilometers across. The mosaic is centered on terrain at 5 degrees south latitude, 85 degrees west longitude. North is up. Credit: NASA/JPL.

Subsurface oceans create astrobiological interest, and we have evidence for various oceans in the outer system, from Europa, Ganymede and Callisto in the Jovian system, and of course in Saturn space on geyser-spewing Enceladus and Titan. In each case, we have an icy surface concealing what may lie below. Perhaps oceans are common in outer system moons, but the situation on Mimas may be different. The paper notes why an ocean there is problematic:

The ocean hypothesis sounds unlikely since no evidence of liquid water, thermal heating or geological activities have been reported on Mimas’ heavily cratered surface, contrary to Enceladus. Radiogenic heating alone could not sustain such an ocean, because the heat produced by the core escapes through the satellite’s icy shell and any ocean would freeze very quickly.

Even so, we can’t dismiss an ocean out of hand. The paper continues:

One explanation could be found in Mimas’ high eccentricity (~0.02), whose origin remains unexplained, and may have been higher in the past. As a consequence, an ocean could have been formed and been sustained due to tidal heating.

If it is there, the researchers think an internal ocean would exist under a crust between 24 and 31 kilometers thick. But what the data show us so far is simply a libration that is twice what we would expect from the moon’s orbital dynamics. This is a value that is being driven primarily by Mimas’ internal structure. The next step in the study of Mimas could proceed, the researchers suggest, by mapping the moon’s gravity field to uncover anomalies, by measuring tidal dissipation using astrometry from Cassini, or by measuring the heat flux at the surface. In the absence of other models to explain these findings, it would appear that Mimas has either an unusual core that is out of equilibrium or an internal ocean beneath thick ice.

All these moons are ours! I like the way that sounds as a derivative of the similar sounding phrase in the second Space Odyssey Sci-Fi flick. A huge amount of resources are available to build starships, which will likely first be space arks sent out eventually in droves with headings all over our corner of the galaxy. I think we will eventually master extreme gamma factor travel, but that will take a big evolution in our power production capacity.

Small moons like Mimas are appealing because of their small gravity well and ease of moving large masses on their surfaces such as in mining and space-ship part fabrications. Obviously, moons in close orbit around the gas giant planets can have quite a gravity well to climb out of but given the 170 or so known moons in our solar system, we have several if not many attractive options.

Could the impactor causing the prominent crater be sufficient to explain the libration, or was this ruled out in the Science paper?

Here’s what the paper says on the subject. Apparently the impactor is not a sufficient explanation:

“Herschel is the largest Mimantean crater; it is located at a longitude of 111.76° W near Mimas’ equator, and is 140 km wide and 10 km deep. The missing material from the impact basin modifies the satellite’s moments of inertia. Moreover, as a consequence of the impact, a large subsurface mass anomaly could have formed below the crater. As in analyses of the effect of the Stickney crater on the libration of Phobos (22, 23), we computed Mimas’ libration amplitude by considering the presence of a large volume with porosity up to 45% (24) at depths as much as 70 km under Herschel (SM5.4). The effect of the basin increases the libration amplitude by about 1 arcmin, which is not enough to explain the observed libration amplitude. On the other hand, the macro porosity increases the libration amplitude by up to 46 arcmin, for which the mass anomaly is so big, that it (if currently present) should cause a reorientation of Mimas about the polar axis by 8°. Such reorientation is inconsistent with the present-day location of Herschel.”

Is it possible that the Herschel impact caused a deformation of the densier core which then became rigid again i.e. froze in place? Prehaps the impact liberated a large amount of volatiles in the mantle which made their way to the surface creating the voids.

Most cratering mechanics text such as “Impact Cratering” by H.J. Melosh conclude central peaks of complex craters are the result of stratigraphic uplift of strata beneath a crater do to compression rebound from the impact event. The central peak of Hershchel is about 6 to 8 km above the crater floor. So the central peak should contain material that was roughly that distance below the surface of Mimas before the cratering event. That would be a good place for a surface lander to investigate.

If Herschel’s morphology isn’t the cause of the libration then maybe Michael is onto something.

We see the surface ‘ripples’ and other lineations on Phobos and more prominently, Vesta, that are linked with large impacts so I wonder if Mimas’ icy crust would behave differently enough to hide such evidence. Could the shockwaves travelling through Mimas deform its core enough to be responsible? If Mimas has partially differentiated then the rockier material deep down may be preserving any deformation rather than the outer shell of ice (can’t remember if we see any surface features near Herschel’s antipode)

Don’t know if this is relevant to my comment above but the Cassini team have an image of Mimas’ northern regions from June 2012 that clearly show some deep, linear ‘trenches’ there… don’t know if they are aligned in the same fashion as the grooving on Vesta but maybe there’s a link to Herschell’s formation?http://astronomynow.com/2014/12/29/polar-scars-on-saturns-moon-mimas/

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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